For example, LNG ships are equipped with LNG storage tanks (liquefied-gas storage tanks) storing LNG (liquefied natural gas). In the LNG storage tanks, incoming heat penetrating tank insulation evaporates the LNG to generate BOG. To keep the internal pressure of the LNG storage tanks constant while preventing a rise in the internal pressure due to the BOG, either a method of releasing the BOG to the outside air or a method of reliquefy the BOG and returning it to the LNG storage tanks is available. Typically used as the method of reliquefy the BOG and returning it to the LNG storage tanks is a method in which BOG taken out from the LNG storage tanks is pressurized by a compressor and is condensed by cooling with cold energy generated by a refrigerator (see PTL 1). A refrigerator used for such applications uses a Brayton cycle using, for example, nitrogen as a primary refrigerant.